The objective of this research is to study the effects of malnutrition early in life on mental development and to establish to what extent it is possible to reverse or prevent intellectual retardation by food supplementation and/or psychological stimulation. Malnutrition is known to be associated with impaired psychological function. This relationship is confounded, however, by health and social factors that covary with nutrition and may themselves affect psychological development. To investigate these complex relationships we have undertaken a prospective multidisciplinary intervention study of children at risk of malnutrition starting at six months of gestation. After sample selection families are assigned at random to six study groups: A) No supplementary feeding; B) Supplementary feeding from six months to 3 years of age; C) Supplementary feeding from 6 months of gestation to 6 months of age; D) Supplementary feeding from 6 months of gestation to 3 years of age; A-1) A home intervention program designed to increase psychological stimulation of the child but no supplementary feeding; D-1) Supplementary feeding from 6 months of gestation to 3 years of age plus psychological stimulation. All groups are given a uniform obstetrical and pediatric health care program. Subjects are observed longitudinally for three years by repeated measurement of nutritional status, intellectual functioning and a number of concomitant social and health variables. This experimental design is expected to yield information on the relative effects of malnutrition at different ages on intellectual functioning. It will also be possible to determine the effects of food supplementation programs of varying duration and starting at different points in life on physical growth, health, and intellectual development.